1. Field of the Invention
This invention concerns a method and apparatus for use in treatment of gynecological tumors with radioactive sources, and more particularly, for treatment of cancer of the uterine cervix.
2. Review of Related Art
Brachytherapy is the use of radioactive materials for the purpose of treatment of cancer. It has a 90-year history, dating to within a few years after the discovery of radium by Marie and Pierre Curie in 1898. It has the unique advantage of delivering high doses (energy/mass) of ionizing radiation to small volumes of tissue, combined with a rapid fall off of dose such that distant anatomy is spared. It thus has provided excellent results for localized control of various cancers.
In brachytherapy, the spatial variation of dose (energy absorbed/tissue mass) around each individual source of radiation varies approximately as 1/r.sup.2, r being the distance from the point of radiation emission to the point of radiation absorption. If more than one radiation source is used, the radiation reaching any particular point is the sum of the radiation emitted from each of the sources. Hence, the dose distribution from a plurality of sources varies anisotropically over the treatment volume.
One of the more successful applications of brachytherapy has been for the treatment of cancer of the uterine cervix. The therapy techniques were developed around sixty years ago with radium and modified with today's usage of cesium-137 and iridium-192. The frequency and severity of complications in treatment of cervical cancer have been correlated with the intracavitary-plus-external-beam dose to the bladder and rectum (Montana, et al., 1989, Int. J. Radiol. Oncol. Biol. Phys., 16:95-100; Perez, et al., 1984, Cancer, 54:235-246; Pourquier, et al., 1982, Int. J. Radiol. Oncol. Biol. Phys., 8:1887-1895); Stockbine, et al., 1970, Am. J. Roentgenol., 108:293-304). Analysis and visualization of dose rates in the body is quite complex, and it is essential to avoid cold (lower dose) spots in the tumor and avoid hot (higher dose) spots in the rectum and bladder. The nominal rules for this treatment are based on evaluation of the dose rate at a few points in the patient (Tod, et al., 1938, Br. J. Radiol., 11:809-823).
The Fletcher system (Delclos, et al., 1980, Int. J. Radiol. Oncol. Biol. Phys., 6:1195-1206; Fletcher, 1953, Radiology, 60:77-84) provides for tandems of varying curvatures to fit in the uterine cavity in various positions, as well as vaginal colpostats with varying ovoid diameters to fit a range of anatomic variations in the vaginal vault (throughout the following "ovoid" shall refer to the closed end of the colpostat nearest the cervix in which the radioactive source is oriented for treatment after insertion). In the Fletcher system, it is preferable to separate the ovoids as far as possible while maintaining their position as close to the vaginal apex as possible. Once satisfied with the separation, the physician utilizes a locking screw to hold the separation after manual release of the colpostats. The largest possible colpostat diameter is usually employed to improve the tumor-to-normal structure dose ratio, as well as to hinder closure of the ovoids (after manual separation) should the locking screw loosen during the several day treatment.
The Fletcher intracavitary applicator (Fletcher, 1953) was developed following the experience of the Manchester tandem and ovoid system (Tod, et al., 1938) for the treatment of cancer of the uterine cervix using radium sources. The ovoids of the Manchester system are mounted on the ends of the colpostats which have bent handles and are used to separate and position the ovoids. This avoided the problem of directly handling the radioactive source carrying ovoids themselves. The original applicator with preloaded radiation sources was modified by Suit et al. (1963, Radiology, 81:126-131) to enable the radiation sources to be afterloaded (inserted into the applicator after it is positioned in the patient), thereby reducing exposure to personnel even further.
Further improvements have been instituted over the years (Delclos, et al., 1980; Delclos, et al., 1978, Cancer, 41:970-979; Delclos, et al., 1970, Radiology, 96:666-667; Green, et al., 1969, Am. J. Roentgenol., 105:609-613; Haas, et al., 1985, Int. J. Radiol. Oncol. Biol. Phys., 11:1317-1321; Haas, et al., 1980, Int. J. Radiat. Oncol. Biol. Phys., 6:1589-1595; Haas, et al., 1983, Int. J. Radiol. Oncol. Biol. Phys., 9:763-768). The development of minicolpostat applicators, such as the Fletcher-Suit-Delclos (FSD) minicolpostats (MC) (Haas, et al., 1983), was particularly significant. The cross-sectional shape of the ovoids is made in the shape of the letter "D" with the flat side oriented medially and perpendicular to the plane of the cervical os opening to the cervix. This narrows the volume of the applicator which must be pushed up through the vagina. This thereby enables the radioactive source to be positioned closer to the cervix while maintaining the largest possible distance from the source to the lateral vaginal walls. Minicolpostats are useful when the vaginal vault is narrow or distorted due to disease. Experience at the Duke University Medical Center shows that minicolpostats are used in over one-third of the cases. Kuske et al. (1988, Int. J. Radiat. Oncol. Biol. Phys., 14:899-906) has reported a similar frequency of use.
In a effort to ensure that the dose distribution will maximize radiation reaching the tumor tissue and minimize radiation reaching the bladder and rectum, the positioning of the applicator in the patient is always verified using plane radiographic films. Further analysis and/or more precise three-dimensional (3D) information on location can potentially be obtained from computed tomography (CT) or magnetic resonance imaging (MRI). However, a common feature in all the Fletcher designs is the use of high-density metal shields to reduce the dose in the direction of the anterior rectal wall and the bladder trigone (typically 20-30% reduction) without decreasing the dose in the direction of the uterosacral and broad ligaments (Delclos, et al., 1978; Fletcher, 1953). This feature of the prior art applicators precludes the obtaining of these 3D imaging modalities for all patients, because the particular disposition of metals in the conventional applicators leads to reconstruction artifacts, obscuring the imaging of the volumes of interest.
To circumvent this problem, Yu et al. (1982, Radiology, 143:536-541) produced a plastic applicator without shields and generated CT images with the applicator in place. Weeks et al. (1989, Endocuriether. Hyperther. Oncol., 5:169-174) have also described a CT-compatible, modified Fletcher applicator system made of plastic with no metal in the applicator (see U.S. Pat. No. 5,012,357). Shields are incorporated in the system of Weeks, et al., 1989, simply by making a container to hold the source and shields together and introducing that container into the applicator after the positioning of the applicator has been verified.
The source+shield container according to U.S. Pat. No. 5,012,357 must then have a much larger diameter than that of just a source carrier. Hence the diameter of the handles of the colpostats must be enlarged to enable the source+shield container to pass through. The handles can no longer be bent without making the handles even larger, but, as the handles get larger it is more difficult to fit the applicator system in patients. Weeks et al. (1991, Int. J. Radiol. Oncol. Biol. Phys., 21:1045-1052) have constructed the smallest possible version of that design, and practical application of that design has established that the applicator can provide treatment for at most 40% of the patient population at Duke Medical Center. However, the considerable amount of operating room time embodied in trying to determine if the applicator will provide an acceptable treatment generally makes such applicators clinically undesirable. Further, it is generally believed that the applicators as described in U.S. Pat. No. 5,012,357 are too large to provide the same level of quality treatment as has been obtained for many years using the metal (steel) FSD design.
New methods of treating cervical cancer have appeared recently. A high activity iridium-192 source (1.5 mm diameter, 5 mm length) is welded on the end of a cable which is motor driven from a protective source housing within a closed end protective plastic sleeve which has been fixed into an applicator inserted in the patient. Because of the high radioactivity of the .sup.192 I source, the treatment lasts only a few minutes and is thus called high dose rate treatment. Applicators for treatment of cervix and vaginal cancer which have copied the FSD shape must be distinct from the low dose rate FSD applicators because of the difficulty of having the source pass by the structure inherently needed to enable the prior art shields stay in place. In short, no shielded applicator exists which could be placed in the patient in the operating room, sustain a standard FSD low dose rate treatment and be followed by a high dose rate boost a day or two later (for the purpose of reducing the overall hospital time that the patient must endure) without putting a different applicator in the patient.